Opioids suppress spontaneous and NMDA-induced inhibitory postsynaptic currents in the dorsal raphe nucleus of the rat in vitro

Tramadol reduces the 5-HTP-induced head-twitch response in mice via the activation of mu and kappa opioid receptors

Tramadol, an atypical opioid analgesic, stimulates both opiatergic and serotonergic systems. Here we have investigated the effect of tramadol in mice on 5-hydroxyptrytophan (5-HTP)-induced head twitch response (HTR), which is an animal model for the activation of the CNS 5-HT(2A) receptors in mice. Tramadol attenuated 5-HTP-induced HTR in a dose-dependent manner as morphine. Furthermore, the nonselective opioid receptor antagonists, naloxone and diprenorphine (M5050), reversed the effect of tramadol on 5-HTP-induced HTR dose-dependently. Interestingly, in contrast to the selective delta opioid receptor antagonist NTI, beta-FNA, a selective mu receptor antagonist, and nor-BNI, a selective kappa opioid receptor antagonist, antagonized the attenuation of 5-HTP-induced HTR by tramadol. In conclusion, administration of tramadol systemically inhibits 5-HTP-induced HTR in mice by activating opiatergic system in the CNS. Our findings show that mu and kappa opioid receptors, but not delta opioid receptor, play an important role in the regulation of serotonergic function in the CNS.

Influence of inhibitory and excitatory inputs on serotonin efflux differs in the dorsal and median raphe nuclei

The dorsal (DRN) and median raphe nuclei (MRN) are two major sources of serotonergic projections to forebrain that are involved in regulation of behavioral state and motor activity, and implicated in affective disorders such as depression and schizophrenia. To investigate afferent influences on serotonergic neurons, this study compared the role of endogenous GABA and glutamate in the DRN and MRN using microdialysis and measurement of locomotor activity in freely behaving rats. Local infusion of the GABA(A) receptor antagonist bicuculline increased serotonin (5-HT) efflux in the DRN but not the MRN. In contrast, infusion of glutamate receptor antagonists produced larger decreases in 5-HT efflux in the MRN compared with the DRN. Moreover, glutamate receptor antagonists attenuated the increase in 5-HT efflux produced by GABA receptor blockade in the DRN. Thus, the disinhibitory effect of GABA blockers could be ascribed in part to an enhanced influence of glutamate. Measurements of locomotor activity indicate that changes in 5-HT were not simply correlated with behavioral activity induced by drug infusion. In summary, the role of inhibitory and excitatory afferents was strikingly different in the DRN and MRN. GABA afferents were the predominant tonic influence on serotonergic neurons in the DRN. In contrast, glutamatergic but not GABAergic afferents had a strong tonic influence on serotonergic neurons in the MRN.

GABAergic and glutamatergic afferents in the dorsal raphe nucleus mediate morphine-induced increases in serotonin efflux in the rat central nervous system

To characterize the effects of morphine on serotonin (5-HT) in the central nervous system, we used microdialysis in freely behaving rats. Subcutaneous injection of morphine sulfate produced a dose-dependent increase in extracellular 5-HT in the dorsal raphe nucleus (DRN) and a forebrain site, the nucleus accumbens (NAcc). To determine the site of action for this effect, the opioid receptor antagonist naltrexone was infused into either the DRN or NAcc. Naltrexone infusion (300 microM) into the DRN but not the NAcc attenuated the increase in 5-HT elicited by systemic morphine (20 mg/kg). This suggests that morphine acts in the DRN to alter the activity of 5-HT neurons that project to NAcc. Consistent with this conclusion, infusion of the GABA(A) receptor antagonist bicuculline (100 microM) into the DRN but not the NAcc also blocked the effect of systemic morphine. Similarly, the effect of systemic morphine was blocked by infusion into the DRN of the GABA(A) receptor agonist muscimol (30 microM) and attenuated by the GABA(B) receptor agonist (+/-)-baclofen (100 microM). This provides evidence that morphine indirectly influences 5-HT release via opioid receptors on GABAergic neurons in the DRN. A new finding is that ionotropic glutamate receptor antagonists [kynurenate or a mixture of (+/-)-2-amino-5-phosphonopentanoic acid and 6,7-dinitro-quinoxaline-2,3-dione] infused in the DRN also attenuated the effect of systemic morphine. These results suggest that morphine acts on GABAergic and glutamatergic afferents to indirectly influence the activity of 5-HT neurons in the DRN. Understanding the details of this neural circuitry may provide new leads for treatment of opiate addiction.

Opioid receptor subtypes differentially modulate serotonin efflux in the rat central nervous system

Opioid receptor subtypes may have site-specific effects and play different roles in modulating serotonergic neurotransmission in the mammalian central nervous system. To test this hypothesis, we used in vivo microdialysis to measure changes in extracellular serotonin (5-hydroxytryptamine; 5-HT) in response to local infusion of mu-, delta-, and kappa-opioid receptor ligands into the dorsal raphe nucleus (DRN), median raphe nucleus (MRN), and nucleus accumbens (NAcc) of freely behaving rats. The mu-opioids [D-Ala(2)-N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), endomorphin-1, and endomorphin-2 were administered by reverse dialysis infusion into the DRN. In response, extracellular 5-HT was increased in the DRN, an effect that was blocked by the selective mu-receptor antagonist beta-funaltrexamine, but not by the delta-receptor antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI-174,864). Infusion of delta-receptor agonists, [D-Ala(2),D-Len(5)]enkephalin (DADLE), [D-Pen(2,5)]enkephalin (DPDPE), and deltophin-II into the DRN also increased extracellular 5-HT, an effect that was blocked by selective delta-receptor antagonists. In contrast to the DRN, local infusion of mu- and delta-opioids had no effect on 5-HT in the MRN or NAcc. These data indicate that mu- and delta-opioid ligands have a selective influence on serotonergic neurons in the DRN. Finally, the kappa-receptor agonist U-50,488 [trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide] produced similar decreases in 5-HT during local infusion into the DRN, MRN, and NAcc. These results provide evidence of differential regulation of 5-HT release by opioid receptor subtypes in the midbrain raphe and forebrain.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Control of the serotonergic system by the medial prefrontal cortex: potential role in the etiology of PTSD and depressive disorders

The prefrontal cortex is involved in an array of higher brain functions that are altered in psychiatric disorders. Serotonergic neurons of the midbrain rapbe nuclei innervate the prefrontal cortex and are the cellular target for drugs used to treat mood disorders such as the selective serotonin (5-HT) reuptake inhibitors. Anatomical evidence supports the existence of projections from the medial prefrontal cortex (mPFC) to the dorsal raphe nucleus (DR). We report on a functional control of the activity of DR 5-HT neurons by projection neurons in the mPFC. The stimulation of the mPFC elicits two types of responses in DR 5-HT neurons, orthodromic excitations and inhibitions. Excitations are mediated by AMPA/KA and NMDA receptors whereas inhibitions are mediated by GABA(A) and 5-HT(1A) receptors. The activation of a subgroup of 5-HT neurons increases 5-HT release which subsequently activates 5-HT(1A) autoreceptors on other 5-HT neurons. GABA(A)-mediated inhibitions involve GABAergic elements in the DR or adjacent areas. Pyramidal neurons of the mPFC co-express postsynaptic 5-HT(1A) (inhibitory) and 5-HT(2A) (excitatory) receptors. Consistent with the above observations, the selective activation of both receptors in mPFC reduced and increased, respectively, the firing activity of DR 5-HT neurons and the 5-HT release in mPFC. Overall, these data indicate that the activity of the 5-HT system is strongly controlled by the mPFC. Thus, the abnormal prefrontal function in post-traumatic stress disorder and depressive patients may induce a disregulation of 5-HT neurons projecting to other brain areas that can underlie the existing symptomatology in these psychiatric disorders.

Integrative role for serotonergic and glutamatergic receptor mechanisms in the action of NMDA antagonists: potential relationships to antipsychotic drug actions on NMDA antagonist responsiveness

NMDA receptor antagonists worsen symptoms in schizophrenia and induce schizophrenic-like symptoms in normal individuals. In animals, NMDA antagonist-induced behavioral responses include increased activity, head weaving, deficits in paired pulse inhibition and social interaction, and increased forced swim immobility. Repeated exposure to NMDA antagonists in animals results in behavioral sensitization-a phenomenon accentuated in rats with dopaminergic neurons lesioned during development. In keeping with an involvement of serotonin and glutamate release in NMDA antagonist action, selected behaviors induced by NMDA antagonists are minimized by 5-HT(2A) receptor antagonists and mGLU2 receptor agonists. These observations provide promising new approaches for treating acute NMDA antagonist-induced psychosis. Further, acute atypical antipsychotic drugs also minimize NMDA antagonist actions to a greater degree than typical antipsychotics. However, because knowledge concerning acute versus chronic effectiveness of various antipsychotic drugs against NMDA antagonist neuropathology is limited, future studies to define more fully the basis of their differences in efficacy after chronic treatment could provide an understanding of their actions on neural mechanisms responsible for the core pathogenesis of schizophrenia.

Cellular basis for the effects of substance P in the periaqueductal gray and dorsal raphe nucleus

Substance P (SP) is known to act at supraspinal sites to influence pain sensitivity as well as to promote anxiety. The effects of SP could be mediated in part by actions in the periaqueductal gray (PAG) and the dorsal raphe nucleus (DRN), adjoining mesencephalic cell groups that are strategically positioned to influence both nociception and mood. Previous studies have indicated that SP regulates both enkephalin and serotonin neurotransmission in these brain regions. To determine the mechanism underlying the effects of SP in the PAG and DRN, the distribution of the principal receptor for SP, the neurokinin 1 (NK1) receptor, was examined with respect to other neurotransmitter markers. PAG neurons that had NK1 receptor immunolabeling were interdigitated with and received contacts from enkephalin-containing neurons. However, only a few (16/144; 11%) neurons with NK1 receptor also contained enkephalin immunoreactivity after colchicine treatment. In the DRN, dendrites containing NK1 receptor were selectively distributed in the dorsomedial subdivision. The majority (132/137; 96%) of these dendrites did not contain immunoreactivity for the serotonin-synthesizing enzyme tryptophan hydroxylase. In contrast, neuronal profiles with NK1 receptor in both the PAG and the DRN often contained immunolabeling for glutamate. Light and electron microscopic examination revealed that 48-65% of cell bodies and dendrites with NK1 receptor were dually immunolabeled for glutamate. These data suggest that SP directly acts primarily on glutamatergic neurons in the PAG and DRN. To a lesser extent, enkephalin-containing neurons may be targeted. Through these actions, it may subsequently influence activity of larger populations of neurons containing enkephalin as well as serotonin. This circuitry could contribute to, as well as coordinate, effects of SP on pain perception and mood.

S-adenosyl-L-methionine prevents 5-HT(1A) receptors up-regulation induced by acute imipramine in the frontal cortex of the rat

S-adenosyl-L-methionine (SAM) has shown efficacy in speeding the onset of the antidepressant effect of imipramine in depressed patients. This effect may be related to their interactions at the serotonin(1A) (5-HT(1A)) receptors. Acute imipramine up-regulated the frontal cortex 5-HT(1A) receptors (B(max), 51.5 +/- 8.4 fmol/mg protein) vs. saline (B(max), 27.5 +/- 5.9 fmol/mg protein), and did not show antidepressant effect. Acute SAM and imipramine+SAM did not modify frontal cortex 5-HT(1A) receptors, and showed antidepressant effects (decrease of the immobility response of 26%, P<0.01; and 47%, P<0.001) vs. saline. All the chronic treatments showed antidepressant effects and up-regulated the hippocampus 5-HT(1A) receptors. SAM prevents the 5-HT(1A) receptor up-regulation induced by acute imipramine in the frontal cortex. This mechanism may contribute to imipramine's antidepressant effect.

Control of dorsal raphe serotonergic neurons by the medial prefrontal cortex: Involvement of serotonin-1A, GABA(A), and glutamate receptors

Anatomical evidence indicates that medial prefrontal cortex (mPFC) neurons project to the dorsal raphe nucleus (DR). In this study, we functionally characterized this descending pathway in rat brain. Projection neurons in the mPFC were identified by antidromic stimulation from the DR. Electrical stimulation of the mPFC mainly inhibited the activity of DR 5-HT neurons (55 of 66). Peristimulus time histograms showed a silence of 150 +/- 9 msec poststimulus (latency, 36 +/- 1 msec). The administration of WAY-100635 and picrotoxinin partly reversed this inhibition, indicating the involvement of 5-HT(1A) and GABA(A) receptors. In rats depleted of 5-HT with p-chlorophenylalanine, the electrical stimulation of mPFC mainly activated 5-HT neurons (31 of 40). The excitations (latency, 17 +/- 1 msec) were antagonized by MK-801 and NBQX. Likewise, MK-801 prevented the rise in DR 5-HT release induced by electrical stimulation of mPFC. The application of 8-OH-DPAT in mPFC significantly inhibited the firing rate of DR 5-HT neurons and, in dual-probe microdialysis experiments, reduced the 5-HT output in mPFC and DR. Furthermore, the application of WAY-100635 in mPFC significantly antagonized the reduction of 5-HT release produced by systemic 8-OH-DPAT administration in both areas. These results indicate the existence of a complex regulation of DR 5-HT neurons by mPFC afferents. The stimulus-induced excitation of some 5-HT neurons by descending excitatory fibers releases 5-HT, which inhibits the same or other DR neurons by acting on 5-HT(1A) autoreceptors. Afferents from the mPFC also inhibit 5-HT neurons through the activation of GABAergic interneurons. Ascending serotonergic pathways may control the activity of this descending pathway by acting on postsynaptic 5-HT(1A) receptors.

Modulation of the locomotor properties of morphine and amphetamine by uncontrollable stress

We have recently demonstrated that exposure to a single session of inescapable shock (IS), but not to identical amounts and distributions of escapable shock (ES), increases the rewarding properties of morphine, as measured by conditioned place preference (CPP). Interestingly, we also found that exposure to IS has no effect, or even interferes with amphetamine CPP. The present study explored whether the potentiating effect of IS on morphine reward, but not amphetamine reward, would generalize to the locomotor properties of these drugs. The locomotor response to morphine and amphetamine was measured 120 h following exposure to either IS or home cage control (HCC) treatment. On test day, the activity of all subjects was measured for 1 h before and 3 h after drug administration. The results demonstrated that exposure to IS potentiated the locomotor response to morphine, while having no effect on the response to amphetamine. An additional study investigated whether the effects of IS on the locomotor properties of morphine were sensitive to stressor controllability, by comparing the influence of IS, ES, or control treatment. Again, IS potentiated the locomotor properties of morphine, while exposure to ES and control treatment had no effect.

Immunoelectron microscopic study of beta-endorphinergic synaptic innervation of GABAergic neurons in the dorsal raphe nucleus

Using a preembedding double immunoreactive technique by immunostaining with antirat beta-endorphin and antisynthetic glutamic acid decarboxylase antisera sequentially, the synaptic relationships between beta-endorphinergic neuronal fibers and GABAergic neurons in the dorsal raphe nucleus of the rat were examined at the ultrastructural level. Although both beta-endorphin-like immunoreactive fibers and glutamic acid decarboxylase-like immunoreactive neurons can be found in the mediodorsal and medioventral parts of the dorsal raphe nucleus, the synapses between them were found only in the mediodorsal part. Most of the beta-endorphin-like immunoreactive neuronal fibers contained many dense-cored vesicles. The synapses made by beta-endorphin-like immunoreactive neuronal axon terminals on glutamic acid decarboxylase-like immunoreactive neurons were both symmetrical and asymmetrical, with the latter predominant, especially in the axo-dendritic synapses. Perikarya with beta-endorphin-like immunoreactivity were found only in the ventrobasal hypothalamus. These findings suggest the possibility that the beta-endorphin-producing neurons in the ventrobasal hypothalamus could influence GABAergic neurons in the dorsal raphe nucleus directly by synaptic relationships.

Effects of opioid receptor activation on cardiovascular responses and extracellular monoamines within the rostral ventrolateral medulla during static contraction of skeletal muscle

During static muscle contraction, activation of opioid receptors alters the extracellular glutamate concentrations within the rostral ventrolateral medulla (RVLM). In addition, microdialysis of glutamate in the ventrolateral medulla (VLM) increases the release of norepinephrine (NE), dopamine (DA), and serotonin (5-HT). Therefore, we hypothesized that extracellular concentrations of these monoamines as well as cardiovascular responses during static skeletal muscle contraction would be modulated following administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the RVLM. Microdialysis of 100 microM DAME into the RVLM of 10 rats significantly (P<0.01) decreased extracellular levels (in pg/10 microl) of NE (from 3.3+/-0.3 to 1.9+/-0.3), DA (from 5.5+/-0.2 to 3.7+/-0.3), and 5-HT (from 6.1+/-0.8 to 3.6+/-0.2) during static exercise. After microdialysis of DAME, the exercise pressor reflex also significantly (P<0.01) decreased mean arterial pressure (MAP) by 13+/-3 mmHg and heart rate (HR) by 16+/-6 bpm, compared with control (MAP=22+/-4 mmHg and HR=31+/-7 bpm). Subsequently, after 30 min microdialysis of naloxone, an opioid receptor antagonist, muscle contraction increased the extracellular monoamine levels (in pg/10 microl, 3.8+/-0.3 NE; 5.2+/-0.3 DA; and 5.5+/-0.4 5-HT) similar to the control groups and evoked a reversal of cardiovascular responses. Similarly, 30 min of microdialyzing naloxone, added to the perfusing medium containing DAME, reversed the attenuating effects of DAME on monoamines, MAP, and HR during a muscle contraction. Furthermore, microdialysis of 100 microM naloxone alone for 30 min potentiated cardiovascular responses and monoamine levels during a muscle contraction. In summary, the present data demonstrates that microdialysis of DAME into RVLM attenuates the exercise pressor reflex mediated increases in MAP, HR and extracellular levels of biogenic monoamines. A subsequent microdialysis of naloxone reversed the effects suggesting that an opioidergic mechanism within RVLM modulates the exercise pressor reflex. Overall, the present study provides further insights into the opioidergic modulation of the exercise pressor reflex.

Corticotropin-releasing factor increases in vitro firing rates of serotonergic neurons in the rat dorsal raphe nucleus: evidence for activation of a topographically organized mesolimbocortical serotonergic system

In vivo studies suggest that the stress-related neuropeptide corticotropin-releasing factor (CRF) modulates serotonergic neurotransmission. To investigate the underlying mechanisms for this interaction, the present study examined the effects of CRF in vitro on dorsal raphe neurons that displayed electrophysiological and pharmacological properties consistent with a serotonergic phenotype. In the presence of either 1 or 2 mm Ca(2+), perfusion of ovine CRF or rat/human CRF rapidly and reversibly increased firing rates of a subpopulation (19 of 70, 27%) of serotonergic neurons predominantly located in the ventral portion of the dorsal raphe nucleus. For a given responsive neuron, the excitatory effects of CRF were reproducible, and there was no tachyphylaxis. Excitatory effects were dose-dependent (over the range of 0.1-1.6 micrometer) and were completely absent after exposure to the competitive CRF receptor antagonists alpha-helical CRF(9-41) or rat/human [d-Phe(12), Nle(21, 38), alpha-Me-Leu(37)]-CRF(12-41). Both the proportion of responsive neurons and the magnitude of excitatory responses to CRF in the ventral portion of the caudal dorsal raphe nucleus were markedly potentiated in slices prepared from animals previously exposed to isolation and daily restraint stress for 5 d. Immunohistochemical staining of the recorded slices revealed close associations between CRF-immunoreactive varicose axons and tryptophan hydroxylase-immunoreactive neurons in the area of the recordings, providing anatomical evidence for potential direct actions of CRF on serotonergic neurons. The electrophysiological properties and the distribution of responsive neurons within the dorsal raphe nucleus are consistent with the hypothesis that endogenous CRF activates a topographically organized mesolimbocortical serotonergic system.

Modulations of Spinal Serotonin Activity Affect the Development of Morphine Tolerance

To test whether modulations of spinal serotonin (5-HT) levels would affect the development of morphine tolerance, we treated rats with either intrathecal 5-HT or 5,7-dihydroxytryptamine (5,7-DHT; a 5-HT neurotoxin) in addition to systemic infusion with morphine (2 mg x kg(-1) x h(-1)). Continuous infusion of 5-HT (10 microg x 6 microL(-1) x h(-1)) into the lumbar subarachnoid space of rats for 9 h accelerated the development of morphine tolerance. The area under the curve for the tail-flick latency test was 454.1 +/- 35.1 in the Sham Control group vs 327.6 +/- 41.0 in the 5-HT-Infused group. mu-opioid receptor binding in the lumbar spinal cord showed a decrease in the Bmax (maximal binding -46.5%), but not the binding affinity (Kd), in 5-HT-infused rats. However, intrathecal injection of 5,7-DHT (50 microg), which resulted in a 48% reduction in 5-HT and 51% reduction in 5-hydroxyindoleacetic acid concentrations, led to an attenuation of morphine tolerance (the area under the curve was 613.0 +/- 24.7 in the 5,7-DHT-Lesioned group). The binding study indicated that the affinity of lumbar micro-opioid receptors decreased 196% in 5-HT-depleted rats, whereas there was no effect on apparent binding. The infusion of 5-HT (10 microg x 6 microL(-1) x h(-1)) was not analgesic and the 5,7-DHT-induced lesion did not affect acute morphine-induced analgesia. We conclude that activity of spinal 5-HT-containing neurons plays a crucial role during the development of morphine tolerance.

Control of serotonergic neurons in rat brain by dopaminergic receptors outside the dorsal raphe nucleus

We studied the control of dorsal raphe (DR) serotonergic neurons by dopaminergic transmission in rat brain using microdialysis and single unit extracellular recordings. Apomorphine (0.5-3.0 mg/kg s.c.) and quinpirole (0.5 mg/kg s.c.) increased serotonin (5-HT) output in the DR and (only apomorphine) in striatum. These effects were antagonized by 0.3 mg/kg s.c. SCH 23390 (in DR and striatum) and 1 mg/kg s.c. raclopride (in DR). 5-HT(1A) receptor blockade potentiated the 5-HT increase produced by apomorphine in the DR. Apomorphine (50-400 microg/kg i.v.) increased the firing rate of most 5-HT neurons, an effect prevented by SCH 23390 and raclopride. Quinpirole (40-160 microg/kg i.v.) also enhanced the firing rate of 5-HT neurons. When applied in the DR, neither drug increased the 5-HT output in the DR or striatum. Likewise, micropressure injection of quinpirole (0.2-8 pmol) failed to increase the firing rate of 5-HT neurons. In situ hybridization showed that the dopamine (DA) D(2) receptor transcript was almost absent in the DR and abundant in the substantia nigra (SN) and the periaqueductal grey matter (PAG). Using dual probe microdialysis, the application of tetrodotoxin or apomorphine in SN significantly increased the DR 5-HT output. Thus, the discrepancy between local and systemic effects of dopaminergic agonists and the absence of DA D(2) receptor transcript in 5-HT neurons suggest that DA D(2) receptors outside the DR control serotonergic activity.

Visual hallucination and tremor induced by sertraline and oxycodone in a bone marrow transplant patient

The authors report a case of probable serotonin syndrome caused by the coadministration of sertraline and oxycodone. A 34 year-old male patient experienced visual hallucinations and severe tremor after dramatically increasing his dosage of oxycodone while on stable amounts of sertraline and cyclosporin. Discontinuation of cyclosporin did not result in resolution of his symptoms. Consideration of a possible sertraline-oxycodone interaction led to withholding sertraline, which resulted in symptom resolution. Serotonin syndrome has been noted with sertraline in combination with other drugs, but this is the first report in combination with a narcotic analgesic. Possible pharmacological mechanisms are discussed. In complicated patients that are taking multiple medications, physicians should be aware of this possible interaction to avoid delay in the diagnosis of serotonin syndrome.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Is there tonic activity in the endogenous opioid systems? A c-Fos study in the rat central nervous system after intravenous injection of naloxone or naloxone-methiodide

This study examined the possibility that a tonic activity in the endogenous opioid systems (EO systems) exists in animals under normal conditions. In a first set of experiments, concurrent changes in behavioral responses and in the numbers of c-Fos-like immunoreactive (Fos-LI) neurons in 58 structures of the brain and lumbosacral spinal cord were analyzed in rats after systemic administration of the opioid antagonist naloxone (NAL; 2 mg/kg). Possible roles of the EO systems were inferred from changes in the numbers of Fos-LI neurons between normal rats that received either NAL or the same volume of saline. Free-floating sections were processed immunohistochemically for c-Fos protein using standard avidin-biotin complex methods. After NAL, the numbers of Fos-LI neurons were significantly increased in the area postrema; in the caudal, intermediate, and rostral parts of the nucleus tractus solitarii; in the rostral ventrolateral medulla; in the Kölliker-Fuse nucleus; in the supramammillary nucleus; and in the central nucleus of the amygdala. In a second set of experiments examining changes in c-Fos expression in the latter structures, similar increases were found after NAL but not after an equimolar dose of NAL-methiodide, a preferential, peripherally acting opioid receptor antagonist. Therefore, Fos-LI was likely triggered after blockade of central opioid receptors, but not peripheral opioid receptors, releasing neurons from EO system-mediated inhibition. The results of this study suggest the existence of a tonic activity of the EO systems exerted on a restricted number of brain regions in normal rats. This tonic activity of the EO systems may control part of the neural networks involved in cardiorespiratory functions and in emotional and learning processes.

Synaptic contacts between serotonergic and cholinergic neurons in the rat dorsal raphe nucleus and laterodorsal tegmental nucleus

We examined synaptic connectivity between cholinergic and serotonergic neurons in the dorsal raphe nucleus and the laterodorsal tegmental nucleus of the rat. To this purpose we employed two variations (the combination of pre-embedding immunogold-silver intensification with avidin-biotin-peroxidase complex technique and the combination of avidin-biotin-peroxidase/3, 3'-diaminobenzidine/silver-gold intensification with avidin-biotin-peroxidase/3,3'-diaminobenzidine reaction) of a double pre-embedding immunoelectron procedure, using primary antibodies against vesicular acetylcholine transporter and serotonin. At the light-microscopic level, serotonin-like immunoreactive neurons in the dorsal raphe nucleus appeared as reddish black and vesicular acetylcholine transporter-like immunoreactive axon terminals were brown colored using a combination of pre-embedding immunogold-silver technique and avidin-biotin-peroxidase complex technique. Serotonin-like immunoreactive fibers projected to the laterodorsal tegmental nucleus. At the electron microscopy level, with both methods we observed in the dorsal raphe nucleus vesicular acetylcholine transporter-immunopositive axon terminals in synaptic contact with serotonin-like immunoreactive dendrites and, to a lesser degree, with serotonin-like immunoreactive cell bodies. These synapses usually were of the symmetrical type. Occasionally we noted, next to vesicular acetylcholine transporter-immunopositive axon terminals, also immunonegative terminals synapsing with the serotonin-like immunoreactive dendrites. In the laterodorsal tegmental nucleus we found serotonin-like immunoreactive axon terminals and immunonegative terminals forming synapses with vesicular acetylcholine transporter-immunoreactive dendrites. Most synapses formed by the serotonin-like immunopositive terminals were of the asymmetrical type. Our results suggest that serotonergic neurons in the dorsal raphe nucleus and cholinergic neurons in the laterodorsal tegmental nucleus may reciprocally influence each other by means of synaptic connectivity. Such connectivity may serve to regulate pain sensation, or be involved in the regulation of the sleeping-waking cycle.

Serotonin 5-HT(2) receptors activate local GABA inhibitory inputs to serotonergic neurons of the dorsal raphe nucleus

The purpose of the present study was to characterize the synaptic currents induced by bath-applied serotonin (5-HT) in 5-HT cells of the dorsal raphe nucleus (DRN) and to determine which 5-HT receptor subtypes mediate these effects. In rat brain slices, 5-HT induced a concentration-dependent increase in the frequency of inhibitory postsynaptic currents (IPSCs) in 5-HT neurons recorded intracellularly in the ventral part of the DRN (EC(50): 86 microM); 5-HT also increased IPSC amplitude. These effects were blocked by the GABA(A) receptor antagonist, bicuculline (10 microM) and by the fast sodium channel blocker, TTX, suggesting that 5-HT had increased impulse flow in local GABAergic neurons. DAMGO (300 nM), a selective mu-agonist, markedly suppressed the increase in IPSC frequency induced by 5-HT (100 microM) in the DRN. A near maximal concentration of the selective 5-HT(2A) antagonist, MDL100,907 (30 nM), produced a large reduction ( approximately 70%) in the increase in IPSC frequency induced by 100 microM 5-HT; SB242,084 (30 nM), a selective 5-HT(2C) antagonist, was less effective ( approximately 24% reduction). Combined drug application suppressed the increase in 5-HT-induced IPSC frequency almost completely, suggesting involvement of both 5-HT(2A) and 5-HT(2C) receptors. Unexpectedly, the phenethylamine hallucinogen, DOI, a partial agonist at 5-HT(2A/2C) receptors, caused a greater increase (+334%) in IPSC frequency than did 5-HT 100 microM (+80%). This result may be explained by an opposing 5-HT(1A) inhibitory effect since the selective 5-HT(1A) antagonist, WAY-100635, enhanced the 5-HT-induced increase in IPSCs. These results indicate that within the DRN-PAG area there may be a negative feedback loop in which 5-HT induces an increase in IPSC frequency in 5-HT cells by exciting GABAergic interneurons in the DRN via 5-HT(2A) and, to a lesser extent, 5-HT(2C) receptors. Increased GABA tone may explain the previous observation of an indirect suppression of firing of a subpopulation of 5-HT cells in the DRN induced by phenethylamine hallucinogens in vivo.

Role and origin of the GABAergic innervation of dorsal raphe serotonergic neurons

Extracellular electrophysiological recordings in freely moving cats have shown that serotonergic neurons from the dorsal raphe nucleus (DRN) fire tonically during wakefulness, decrease their activity during slow wave sleep (SWS), and are nearly quiescent during paradoxical sleep (PS). The mechanisms at the origin of the modulation of activity of these neurons are still unknown. Here, we show in the unanesthetized rat that the iontophoretic application of the GABA(A) antagonist bicuculline on dorsal raphe serotonergic neurons induces a tonic discharge during SWS and PS and an increase of discharge rate during quiet waking. These data strongly suggest that an increase of a GABAergic inhibitory tone present during wakefulness is responsible for the decrease of activity of the dorsal raphe serotonergic cells during slow wave and paradoxical sleep. In addition, by combining retrograde tracing with cholera toxin B subunit and glutamic acid decarboxylase immunohistochemistry, we demonstrate that the GABAergic innervation of the dorsal raphe nucleus arises from multiple distant sources and not only from interneurons as classically accepted. Among these afferents, GABAergic neurons located in the lateral preoptic area and the pontine ventral periaqueductal gray including the DRN itself could be responsible for the reduction of activity of the serotonergic neurons of the dorsal raphe nucleus during slow wave and paradoxical sleep, respectively.

Regulation of serotonin release by GABA and excitatory amino acids

Regulation of serotonin release by gamma-aminobutyric acid (GABA) and glutamate was examined by microdialysis in unanaesthetized rats. The GABA(A) receptor agonist muscimol, or the glutamate receptor agonists kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolaproprionate or N-methyl-D-aspartate were infused into the dorsal raphe nucleus (DRN) while extracellular serotonin was measured in the DRN and nucleus accumbens. Muscimol produced decreases, and the glutamate receptor agonists produced increases in serotonin. To determine if these receptors have a tonic influence on serotonergic neurons, glutamate or GABA(A) receptor antagonists were infused into the DRN. Kynurenate, a nonselective glutamate receptor blocker, produced a small, 30% decrease in serotonin. A similar decrease was obtained with combined infusion of AP-5 and DNQX into the DRN. The GABAA receptor blocker bicuculline produced an approximately three-fold increase in DRN serotonin. In conclusion, glutamate neurotransmitters have a weak tonic excitatory influence on serotonergic neurons in the rat DRN. However, the predominate influence is mediated by GABA(A) receptors.

Dual control of dorsal raphe serotonergic neurons by GABA(B) receptors. Electrophysiological and microdialysis studies

We assessed the role of GABA(B) receptors in the control of serotonergic (5-HT) neurons of the dorsal raphe nucleus (DRN) by using microdialysis in vivo and intra- and extracellular recording in vitro in the rat. The GABA(B) agonist R(+)baclofen (but not the inactive S(-)enantiomer) enhanced the 5-HT output in the DRN (4. 7-fold at 15 mg/kg s.c.) and, to a much lesser extent, striatum of unanesthetized rats. Phaclofen (2 mg/kg s.c.) antagonized the effects of 6 mg/kg R(+)baclofen in dorsal striatum. Using dual-probe microdialysis, R(+)baclofen (0.1-100 microM) applied in the DRN enhanced the local 5-HT output (4.5-fold at 100 microM) but decreased that in striatum at 100 microM. At concentrations higher than 100 microM there was a moderate decrement in the elevation of 5-HT in the DRN. In midbrain slices, bath R(+)baclofen exerted a biphasic effect on DRN 5-HT neurons. Consistent with a reduced striatal 5-HT release when infused in the DRN, R(+)baclofen (0.1-30 microM) induced an outward current in 5-HT neurons (IC(50) = 1.4 microM). Lower R(+)baclofen concentrations (0.01-1 microM) preferentially reduced GABAergic inhibitory postsynaptic currents induced by N-methyl-D-aspartate (20 microM) in 5-HT neurons (IC(50) = 72 nM). Using extracellular recordings, R(+)baclofen (300 nM) enhanced the ability of NMDA to induce firing in a subpopulation of serotonergic neurons. These results are consistent with a preferential activation by a low concentration of R(+)baclofen of presynaptic GABA(B) receptors on GABAergic afferents that could disinhibit 5-HT neurons and increase 5-HT release.

Chronic morphine increases GABA tone on serotonergic neurons of the dorsal raphe nucleus: association with an up-regulation of the cyclic AMP pathway

Major adaptations after chronic exposure to morphine include an up-regulation of the adenosine 3',5'-monophosphate pathway. Acute opioids, via mu-opioid receptors, disinhibit midbrain serotonergic neurons by suppressing inhibitory GABAergic transmission in the dorsal raphe nucleus and adjacent periaqueductal gray. This study examined whether chronic morphine induces a compensatory increase in GABA inputs to 5-hydroxytryptamine neurons and whether this was associated with an up-regulation of the adenosine 3',5'-monophosphate pathway. The firing rate of serotonergic neurons was reduced in brain slices from morphine-dependent rats, an effect reversed by the GABA(A) antagonist bicuculline. The reduction in firing rate was accompanied by an increased frequency of spontaneous GABAergic inhibitory postsynaptic currents, indicating increased GABA tone in the slice. The increase in GABA tone in brain slices from dependent rats was associated with increased induction of inhibitory postsynaptic currents by the adenylyl cyclase activator forskolin, suggesting an up-regulation of the adenosine 3',5'-monophosphate pathway. Indeed, chronic morphine increased levels of adenylyl cyclase VIII (but not of adenylyl cyclase I, III or V) immunoreactivity in the dorsal raphe nucleus area. Two adenosine 3',5'-monophosphate-mediated mechanisms for the increase in GABA tone were discerned. The first, which predominated when impulse-flow was blocked by tetrodotoxin, involves protein kinase A since it was sensitive to protein kinase A inhibitors. The second, seen when impulse-flow was intact (i.e. absence of tetrodotoxin), was insensitive to protein kinase A inhibitors but was suppressed by ZD7288, a blocker of hyperpolarizing-activated Ih channels which are directly activated by adenosine 3',5'-monophosphate. We conclude that chronic morphine induces an up-regulation of the adenosine 3',5'-monophosphate pathway in GABAergic inputs to serotonergic cells, resulting in an increase in spontaneous and impulse-flow dependent GABA release. These changes would lead to an increase in GABA tone and subsequently to the reported decrease in serotonergic activity during opiate withdrawal.

Effects of tamoxifen on serotonin transporter and 5-hydroxytryptamine(2A) receptor binding sites and mRNA levels in the brain of ovariectomized rats with or without acute estradiol replacement

Estradiol-17beta (E(2)), in its positive feedback mode for gonadotropin release in the female rat, induces expression of the genes for the 5-hydroxytryptamine(2A) receptor (5-HT(2A)R) and the serotonin transporter (SERT) in the dorsal raphe nucleus (DRN) with a concomitant increase in the densities of 5-HT(2A)R and the SERT in rat forebrain. The forebrain regions affected are those which, in humans, are concerned with the control of mood, mental state, cognition and emotion. Here we have used the mixed estradiol agonist/antagonist, tamoxifen, to determine whether this action of estradiol is mediated by cytoplasmic estradiol receptors. Acute treatment ( approximately 32 h) of ovariectomized rats with estradiol benzoate (EB) increased significantly the amount of 5-HT(2A)R mRNA and SERT mRNA in the DRN and the densities of 5-HT(2A)R and SERT binding sites in the forebrain. These effects of EB were completely blocked by tamoxifen. Treatment with tamoxifen alone had no effect on either gene expression or the density of binding sites. Together, these data show that tamoxifen acts as a pure estradiol antagonist with respect to serotonergic mechanisms in brain. Detailed analysis of the effects of estradiol and tamoxifen on the DRN showed that SERT gene expression is constitutive only in the posterior DRN; in the anterior DRN, SERT gene expression appears to depend upon estrogen induction which is blocked by tamoxifen. Our findings strongly suggest that estradiol receptors are involved in mediating estradiol action on central serotonergic mechanisms and are relevant for our understanding of the effects of antiestrogens as well as estradiol on mood, mental state and cognition.

Androgen actions on central serotonin neurotransmission: relevance for mood, mental state and memory

Sex steroids exert potent effects on mood and mental state in the human. Our previous experimental findings in female rats suggest that these effects may be mediated, in part, by the action of estrogen on the 5-hydroxytryptamine2A receptor (5-HT(2A)R) and serotonin transporter (SERT) in brain. Here we review our recent findings on the effect of acute (approximately 32 h) testosterone manipulation on central 5-HT(2A)R and SERT in male rats. Castration decreased while testosterone or estrogen, but not 5alpha-dihydrotestosterone (5alpha-DHT), increased significantly the content of 5-HT(2A)R mRNA and SERT mRNA in the dorsal raphe nucleus (DR) and the density of 5-HT(2A)R and SERT binding sites in higher centers of the brain. The lack of effect of 5alpha-DHT, a potent androgen which cannot be converted to estrogen, suggests that the action of testosterone depends upon its conversion to estrogen by aromatase. This may also explain why estrogen, but not testosterone or 5alpha-DHT, increased the density of 5-HT(2A)R binding sites in the caudate-putamen, a brain region where aromatase is scarce. The estrogen induction of SERT mRNA is most prominent in the rostral DR and this together with the correlation between sensitivity of DR serotonin neurons to estrogen and neurotoxic amphetamine derivatives provides a potential topochemical handle with which to investigate testosterone/estrogen regulation of SERT gene expression. These findings are discussed in relation to the possible role of interactions between sex steroids and serotonin mechanisms in mood disorders, schizophrenia and Alzheimer's disease.

Neuropeptide FF selectively attenuates the effects of nociceptin on acutely dissociated neurons of the rat dorsal raphe nucleus

Intracellular Ca2+ concentration ([Ca2+]i) was measured in neurons, acutely dissociated from the rat dorsal raphe nucleus (DRN), with the fluorescent calcium probe Fluo3. Nociceptin (300 nM) had no effect on resting [Ca2+]i but reduced the magnitude of the [Ca2+]i transient triggered by depolarization in 90% of neurons having polygonal or fusiform perikarya. In 94% of neurons with the same morphology 5-HT (30 microM) also reduced the magnitude of the [Ca2+]i transient. The selective 5-HT(1A) receptor antagonist 4-iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-ben zamide hydrochloride (p-MPPI) (0.4 microM) strongly attenuated (by 72+/-7%, n=4) this effect. The responses to nociceptin and 5-HT were not affected by BaCl2 (100 microM). The neuropeptide FF analog [D-Tyr1, (N-Me)Phe3]NPFF (1DMe) altered neither the resting [Ca2+]i nor the [Ca2+]i transient triggered by depolarization but dose-dependently decreased the effect of nociceptin (EC50=1.8 nM, maximal reduction: 68+/-5%). 1DMe had no effect on the response to 5-HT. Another neuropeptide FF analog, exhibiting a different pharmacological activity in mice and rats, [D-Tyr1, D-Leu2, D-Phe3]NPFF (1 microM) also reduced the effect of nociceptin by 74+/-11% (n=4). Few neurons (5 out of 42), either with polygonal/fusiform or smaller ovoid cell bodies, responded to the mu-opioid receptor agonist [D-Ala2, (N-Me)Phe4, Gly-ol5]-enkephalin (DAGO) with a decrease in the depolarization-induced [Ca2+]i transient. 1DMe (100 nM) attenuated this response by 69+/-14%. These results suggest that, at the cellular level, neuropeptide FF selectively counteracts the effects of opioid receptor activation.

Role of the dorsal raphe nucleus in morphine-induced immediate early gene expression in the rat striatum

Serotonin (5-HT) is thought to be involved in morphine action in the brain. To determine if the periaqueductal gray (PAG) and the dorsal raphe nucleus (DRN) are involved in morphine-induced c-Fos and JunB expression in the caudate-putamen (CPu), the mu receptor antagonist, beta-funaltrexamine (beta-FNA), was unilaterally infused into the PAG adjacent to DRN prior to morphine. Behaviorally, beta-FNA prevented morphine-induced loss of righting and Straub tail. In the CPu of beta-FNA treated rats, morphine-induced c-Fos and JunB were attenuated compared to vehicle-infused rats. These results suggest that morphine acts within the PAG-DRN to exert rapid behavioral effects and to induce c-Fos and JunB in the striatum.

Infusion of beta-FNA into the thalamus attenuates morphine-induced c-Fos induction in the rat caudate putamen

The medial thalamus contains mu opioid receptors and sends a glutamatergic projection to the caudate putamen (CPu) in rat. Morphine-induced c-Fos expression in the CPu has been shown to be blocked by pretreatment with antagonists to N-methyl-D-aspartate receptors, indicating the involvement of glutamate in this morphine-induced response. The importance of the glutamatergic projections from the thalamus was assessed by infusing the mu opioid receptor antagonist, beta-funaltrexamine (beta-FNA), prior to systemic morphine injection. Infusion of beta-FNA near specific medial thalamic nuclei attenuated morphine-induced c-Fos expression in the CPu.

Origins of the glycinergic inputs to the rat locus coeruleus and dorsal raphe nuclei: a study combining retrograde tracing with glycine immunohistochemistry

The amino acid glycine is a major inhibitory neurotransmitter in the brainstem and is likely involved in the tonic inhibition of the monoaminergic neurons during all sleep-waking stages. In order to determine the neurons at the origin of the glycinergic innervation of the two principal monoaminergic nuclei, the locus coeruleus and the dorsal raphe of the rat, we applied a double-labelling technique, combining retrograde transport of cholera-toxin B subunit with glycine immunohistochemistry. Using this technique, we found that the locus coeruleus and dorsal raphe nuclei receive a common glycinergic innervation from the ventral and ventrolateral periaqueductal grey, including the adjacent deep mesencephalic reticular nucleus. Small additional glycinergic inputs to these nuclei originated from the lateral paragigantocellular nucleus and the rostral ventromedial medullary reticular formation. The potential role of these glycinergic inputs in the control of the excitability of the monoaminergic neurons of the locus coeruleus and dorsal raphe nuclei is discussed.

Opioid-dependent effects of inescapable shock on escape behavior and conditioned fear responding are mediated by the dorsal raphe nucleus

Manipulations of the dorsal raphe nucleus (DRN) modulate the behavioral effects of exposure to inescapable shock (IS). Opiate agonists and antagonists also influence the impact of IS, but the role of the DRN in mediating these effects is unknown. The opiate antagonist naltrexone micro-injected into the region of the DRN immediately prior to IS prevented both the escape deficit and the enhancement of fear conditioning that occur 24 h later. Intra-DRN naltrexone administered at the time of later behavioral testing reduced, but did not eliminate, these effects of prior IS. Conversely, the opiate agonist morphine, in combination with a subthreshold number of 20 IS trials, induced an escape deficit and enhanced conditioned fear 24 h later. Microinjections of naltrexone into the dorsolateral periaqueductal gray area did not alter the effects of IS and electrolytic lesions of the DRN prevented the effect of the morphine-20 IS trial combination. The role of opioids in mediating the behavioral effects of IS is discussed.

Serotonin transporter (SERT) mRNA and binding site densities in male rat brain affected by sex steroids

Estrogen increases serotonin transporter (SERT) mRNA and binding sites in female rat brain. In order to determine whether changes in SERT are gender- and steroid-specific we have now carried out studies on adult male Wistar rats which were either intact or castrated (under halothane anesthesia) and injected with arachis oil, estradiol benzoate (EB), testosterone propionate (TP) or the non-aromatizable androgen, 5alpha-dihydrotestosterone (5alpha-DHT). The number of SERT mRNA-expressing cells in the dorsal raphe (DR) nucleus was decreased by castration and increased by treatment (for approximately 32 h) with EB or TP, but not 5alpha-DHT. Sex steroids had no effect on the number of SERT mRNA-expressing cells in the median raphe nucleus. The density of SERT sites, assessed by autoradiography of [3H]paroxetine binding, was significantly reduced in arcuate nucleus and median raphe after castration, and increased in arcuate, basolateral amygdala and ventromedial hypothalamic nucleus by treatment with EB or TP, but not 5alpha-DHT. Estradiol, but not testosterone or 5alpha-DHT reduced the density of SERT sites in midbrain central grey. These data show that testosterone as well as estrogen affects SERT expression in male brain, and that the action of testosterone probably depends upon its enzymatic conversion, by aromatase, to estradiol. Our findings may have implications for sex steroid control of mood and behavior, and the action of neurotoxic derivatives of amphetamine, such as 3, 4-methylenedioxymethamphetamine, in the human.

An electrophysiological and neuroanatomical study of the medial prefrontal cortical projection to the midbrain raphe nuclei in the rat

In this study we utilized electrophysiological and pathway tracing methods to investigate the projections from the medial prefrontal cortex to the midbrain raphe nuclei of the rat. Initial pathway tracing experiments using retrograde (horseradish peroxidase conjugates with wheatgerm agglutinin or choleratoxin B subunit) and anterograde (Phaseolus vulgaris-leucoagglutinin) markers demonstrated a direct, bilateral projection to the dorsal raphe nucleus and median raphe nucleus from the medial prefrontal cortex, and the origin of this projection was localized predominantly in the ventral medial prefrontal cortex (infralimbic/dorsal penduncular cortices). Using chloral hydrate-anaesthetized rats, extracellular recordings were made mostly from 5-hydroxytryptamine neurons in the dorsal raphe nucleus, but non-5-hydroxytryptamine dorsal raphe neurons were also studied, as was a small number of 5-hydroxytryptamine neurons in the median raphe nucleus. In an initial study, electrical stimulation of the ventral medial prefrontal cortex caused a post-stimulus inhibition in the majority (49/56) of dorsal raphe 5-hydroxytryptamine neurons tested (mean duration of inhibition, 200+/-17 ms); in some cases (8/56) the inhibition was preceded by short-latency (26 +/-3 ms) orthodromic activation, and a small number of cells was antidromically activated (6/56). Both single spiking and burst-firing 5-hydroxytryptamine neurons in the dorsal raphe nucleus responded in the same way, and median raphe 5-hydroxytryptamine neurons were also inhibited (5/5). In contrast, few (2/12) of the non-5-hydroxytryptamine dorsal raphe neurons tested were inhibited by ventral medial prefrontal cortex stimulation. The effects of stimulation of the dorsal and ventral medial prefrontal cortex were compared on the same raphe 5-hydroxytryptamine neurons (n=17): ventral medial prefrontal cortex stimulation inhibited 16/17 of these neurons while only 8/17 were inhibited by dorsal medial prefrontal cortex stimulation. Finally, the inhibitory effect of ventral medial prefrontal cortex stimulation on 5-hydroxytryptamine cell-firing was not altered by 5-hydroxytryptamine depletion with p-chlorophenylalanine or by systemic administration of the selective 5-hydroxytryptamine1A receptor antagonist WAY 100635. The latter findings indicate that the inhibition is not due to release of raphe 5-hydroxytryptamine which could theoretically arise from anti- or orthodromically activated 5-hydroxytryptamine neurons. Our results show that stimulation of the ventral medial prefrontal cortex causes a marked post-stimulus inhibition in the vast majority of midbrain raphe 5-hydroxytryptamine neurons tested. It seems likely that the projection from ventral medial prefrontal cortex to the midbrain raphe nuclei mediates the responses of 5-hydroxytryptamine neurons to cortical stimulation. These data are relevant to recent discoveries of functional and structural abnormalities in the medial prefrontal cortex of patients with major depressive illness.

Pre- and postsynaptic inhibitory actions of methionine-enkephalin on identified bulbospinal neurons of the rat RVL

The effects of methionine-enkephalin (ME) on visualized bulbospinal neurons of the rostral ventrolateral medulla (RVL) were characterized in thin slices at 32 degrees C using the whole cell patch-clamp technique. Thirty-five percent of the recorded neurons were found to be tyrosine hydroxylase immunoreactive (C1 neurons). In voltage-clamp recordings, ME (3 microM) induced an outward current in 66% of RVL bulbospinal neurons. A similar percentage of C1 and non-C1 neurons were opioid sensitive. The current induced by ME was inwardly rectifying, reversed close to the potassium equilibrium potential, and was blocked by barium. Most spontaneous postsynaptic currents recorded in these neurons were tetrodotoxin (TTX)-resistant miniature postsynaptic currents (mPSCs). Approximately, 75% of mPSCs had rapid kinetics (decay time = 4.7 ms) and were glutamatergic [miniature excitatory postsynaptic currents (mEPSCs)] because they were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). The remaining mPSCs had much slower kinetics (decay time = 19.6 ms) and were GABAergic [miniature inhibitory postsynaptic currents (mIPSCs)] as they were blocked by gabazine (3 microM) but not by strychnine (3-10 microM). ME decreased the frequency of mEPSCs and mIPSCs by 69 and 43%, respectively. The inhibitory effects of ME were mimicked by the selective mu-opioid receptor agonist endomorphin-1 (EM, 3 microM) and were blocked by naloxone (1 microM). In the absence of TTX, excitatory PSCs evoked by focal electrical stimulation were isolated by application of gabazine and strychnine. EM reduced the amplitude of the evoked EPSCs by 41% without changing their decay time. We conclude that opioids inhibit the majority of RVL C1 and non-C1 bulbospinal neurons by activating a potassium conductance postsynaptically and by decreasing the presynaptic release of glutamate. These cellular mechanisms could explain the depressive cardiovascular effects and the sympathoinhibition produced by opioid transmitters in the RVL, in particular during hypotensive hemorrhage.

Morphological substrates underlying opioid, epinephrine and gamma-aminobutyric acid inhibitory actions in the rat locus coeruleus

The locus coeruleus (LC) has been implicated in attentional processes related to orienting behaviors, learning and memory, anxiety, stress, the sleep-wake cycle, and autonomic control, as well as to contributing to the affective state. Direct activation of LC neurons causes desynchronization of the electroencephalogram, suggesting that the LC is an important modulator of the behavioral state. The LC has been an intensely studied neuronal system, as the physiology and pharmacology of this nucleus is well understood. This is mainly because of the similarity in neurochemical composition of LC cells which all contain norepinephrine in the rat. However, the homogeneity in neurotransmitter content in LC neurons is sharply contrasted by the heterogeneity of neurochemicals found in its afferent processes. Among these are axon terminals that contain inhibitory and excitatory amino acids, monoamines, and neuropeptides, many of which have been shown to exert differential physiological effects on LC discharge activity. Although much attention has focused on physiological activation of LC neurons, substantial evidence indicates that diverse afferents prominently inhibit noradrenergic cellular activity. Such inhibitory neurochemicals, which arise from local and extrinsic sources, include gamma-aminobutyric acid (GABA) and epinephrine as well as the neuropeptides methionine5-enkephalin and leucine5-enkephalin. Inhibitory transmission in the LC has widespread implications for norepinephrine release at diverse postsynaptic targets, and clinically useful pharmacological agents such as clonidine, an alpha2 adrenergic receptor agonist that potently inhibits the firing of LC neurons, alleviate some negative physical symptoms observed following withdrawal from opiates. In the present review, the synaptic and functional organization of selected inhibitory-type neurotransmitters in the LC obtained from immunoelectron microscopic data will be discussed.

Testosterone as well as estrogen increases serotonin2A receptor mRNA and binding site densities in the male rat brain

Our previous findings in female rats suggest that the potent effects of sex steroids on mood and mental state may be mediated, in part, by the effect of estrogen on the 5-hydroxytryptamine2A receptor (5-HT2AR) in brain. The aim of the present study was to determine the effect of acute (approximately 32h) sex steroid manipulation on central 5-HT2AR in the adult male Wistar rat. Castration (under halothane anesthesia) decreased while testosterone or estrogen, but not 5alpha-dihydrotestosterone (5alpha-DHT), increased significantly the 5-HT2AR mRNA content in dorsal raphe nucleus and the density of 5-HT2AR binding sites in frontal, cingulate and primary olfactory cortex and nucleus accumbens. The lack of effect of 5alpha-DHT, a potent androgen which cannot be converted to estrogen, suggests that the action of testosterone depends upon its conversion to estrogen by aromatase. This may also explain why estrogen, but not testosterone or 5alpha-DHT, increased the density of 5-HT2AR binding sites in the caudate-putamen, a brain region where aromatase is scarce. These findings are discussed in relation to the possible role of the 5-HT2AR in depression, schizophrenia and Alzheimer's Disease.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Opposite modulation of opiate withdrawal behaviors on microinfusion of a protein kinase A inhibitor versus activator into the locus coeruleus or periaqueductal gray

Chronic opiate administration upregulates the cAMP pathway in the locus coeruleus (LC). This adaptation is thought to increase the electrical excitability of LC neurons and contribute to the dramatic increase in LC firing induced by opioid receptor antagonists in opiate-dependent animals. The goal of the present study was to evaluate directly a role of the cAMP pathway in opiate withdrawal behaviors by studying, in vivo, whether withdrawal is influenced by intra-LC infusion of compounds known to activate or inhibit protein kinase A (PKA). Infusions into amygdala or periaqueductal gray (PAG) were studied for comparison. In one series of experiments the effect of intra-LC, intra-amygdala, or intra-PAG infusions of the PKA inhibitor Rp-cAMPS on naloxone-precipitated withdrawal from morphine was examined. Intra-LC infusions of Rp-cAMPS significantly attenuated several prominent behavioral signs of morphine withdrawal. Intra-PAG infusions of Rp-cAMPS also significantly attenuated opiate withdrawal behaviors, although different behaviors were affected. In contrast, intra-amygdala infusions of Rp-cAMPS were without significant effect. In a second series of experiments the effect of intra-LC or intra-PAG infusions of the PKA activator Sp-cAMPS on behavior in nondependent drug-naive animals was determined. Sp-cAMPS infusions into either brain region induced a quasi-withdrawal syndrome, but the observed behaviors differed between the two groups. Analysis of the phosphorylation state of tyrosine hydroxylase, a well characterized substrate for PKA, confirmed the ability of Rp-cAMPS and Sp-cAMPS to inhibit and activate, respectively, PKA activity in vivo. Together, these data provide direct evidence for involvement of the cAMP-PKA system in the LC, as well as in the PAG, in opiate withdrawal and withdrawal-related behaviors.

Molecular and cellular basis of addiction

Drug addiction results from adaptations in specific brain neurons caused by repeated exposure to a drug of abuse. These adaptations combine to produce the complex behaviors that define an addicted state. Progress is being made in identifying such time-dependent, drug-induced adaptations and relating them to specific behavioral features of addiction. Current research needs to understand the types of adaptations that underlie the particularly long-lived aspects of addiction, such as drug craving and relapse, and to identify specific genes that contribute to individual differences in vulnerability to addiction. Understanding the molecular and cellular basis of addictive states will lead to major changes in how addiction is viewed and ultimately treated.

Neurotensin and the serotonergic system

The serotonergic system, because of very diffuse projections throughout the central nervous system, has been implicated in numerous functions including nociception, analgesia, sleep-wakefulness and autonomic regulation. Despite an abundant literature indicating the presence of neurotensin-containing (neurotensinergic) neurons, fibres and terminals in most areas containing serotonergic neurons, little is known about the possible relationship between serotonergic and neurotensinergic systems. The purpose of this review is (i) to summarize current knowledge on the anatomical relation between neurotensinergic and serotonergic system, (ii) to summarize current knowledge on the action of neurotensin on serotonergic neurons and (iii) to discuss the possible physiological relevance of this action. Neurotensin-containing cell bodies can be found in the most rostral raphe nuclei. There are neurotensin-containing fibres and terminals in all raphe nuclei. Raphe nuclei have also been shown to contain neurotensin-receptor binding sites. In the dorsal raphe nucleus, neurotensin induces a concentration-dependent increase in the firing rate of a subpopulation of serotonergic neurons. The neurotensin-induced excitation, which is selectively blocked by the non-peptide neurotensin receptor antagonist SR 48692, is observed mainly in the ventral part of the nucleus. Most serotonergic neurons show marked desensitization to neurotensin, even at low concentrations. In intracellular experiments, neurotensin induces an inward current, associated in some cases with a decrease in apparent input conductance, which is occluded by supramaximal concentrations of the alpha 1-adrenoceptor agonist phenylephrine. In rare cases, neurotensin induces an excitation of GABAergic or glutamatergic neurons. Since the neurotensinergic system has also been implicated in nociception, analgesia, sleep-wakefulness, and autonomic regulation, the review discusses the possibility that part of this regulation could involve the activation of the serotonergic system.